Electron multiplier

ABSTRACT

The present invention relates to an electron multiplier including a structure which effectively prevents an electric field from leaking to the front side of an MCP due to a voltage applied to the MCP, facilitates an operation of attaching/detaching the MCP, and prevents the MCP from being damaged and so forth during the operation. The electron multiplier includes an MCP as electron multiplying means, and an outer electric field shield member for accommodating the MCP. The electric field shield cap includes a body portion surrounding at least a side face of the MCP. The electron multiplier further includes a structure by which, while the MCP is held by the outer electric field shield member and an inner electric field shield member accommodated in the outer electric field shield member, the outer electric field shield member is attached to a base of the electron multiplier so as to facilitate the operation of attaching/detaching the MCP.

DESCRIPTION

1. Technical Field

The present invention relates to a device applicable, for example, toelectron microscopes; and, in particular, to an electron multiplier formultiplying incident electrons and outputting thus multiplied electrons.

2. Background Art

As conventional electron microscopes, those of reflection type andtransmission type have been known. To these electron microscopes, anelectron multiplier equipped with a microchannel plate (hereinafterreferred to as MCP) having a plurality of channels for electronmultiplication is applicable as electron multiplying means.

Here, the reflection type electron microscope is an apparatus whichdetects secondary electrons from a sample generated by an electron beamemitted toward the sample, and processes the resulting signal, so thatan image of the sample is observed. On the other hand, the transmissiontype electron microscope is an apparatus which detects electronstransmitted through a sample irradiated with an electron beam, so thatan image of the sample is observed.

The electron beam apparatus disclosed in Japanese Patent ApplicationLaid-Open No. 2-275368 and the like, for example, have been known as theprior art related to the above-mentioned reflection type electronmicroscope, whereas the electron beam apparatus disclosed in JapanesePatent Application Laid-Open No. 6-310076 and the like, for example,have been known as the prior art related to the above-mentionedtransmission type electron microscope.

DISCLOSURE OF THE INVENTION

As a result of studies of conventional electron microscopes, theinventors have found the following problems. Namely, with the reflectiontype electron microscope, transmitted electron images formed byelectrons transmitted through a sample cannot be observed. In otherwords, though a surface of the sample can be observed therewith, theinner structure of the sample and the like cannot be observed, wherebyseparate transmission type viewing equipment is necessary for observingsuch an inner structure and the like.

With the transmission scanning electron beam apparatus, on the otherhand, while a transmitted electron image formed by electrons transmittedthrough a sample can be observed by an electron detector installed belowthe sample (in a space opposing the space where an electron source isinstalled with respect to the sample), and a reflected electron imageformed by secondary electrons from the sample can be observed by anelectron detector located above the sample, the dedicated electrondetectors are necessary for obtaining the respective electron images.Also, a signal selecting circuit for selecting the respective signals ofthe electron detectors is necessary along therewith. Consequently, theapparatus becomes complicated, thereby increasing the size and costthereof.

For example, in cases where a voltage to an MCP (electron multiplyingmeans) of an electron multiplier employed in such an electron microscopeis selectively supplied and stopped, so that the reflected electronimage and transmitted electron image can be observed alternately, anelectric field may leak to the front side of the MCP when the voltage issupplied thereto, whereby the electron beam incident on the sample maybe bent by the leakage electric field. Due to this phenomenon, each partof the resulting transmitted electron image shifts to a positiondifferent from that of the reflected electron image. According to theexperiments conducted by the inventors, such electron image shiftingoccurred by about 3 μm when a voltage of 700 V was applied to the MCP,whereby the amount of shift between the electron images would not benegligible anymore when observed at a high magnification (e.g., 1 μm×1μm).

In addition, when it comes to the handling of the electron multiplierequipped with the MCP, it has conventionally been necessary for askilled person to take considerable care in replacing the MCP in theelectron multiplier in order to prevent the MCP from being damaged orcontaminated. In particular, the conventional structure for mounting theelectron multiplier has such a configuration that the MCP unit is hardto remove therefrom, whereby it has been necessary to replace the wholeelectron multiplier in its already mounted apparatus.

In order to overcome the problems such as those mentioned above, it isan object of the present invention to provide an electron multipliercomprising a structure which effectively prevents an electric field fromleaking to the vicinity of an MCP due to a voltage applied to the MCP,facilitates an operation of attaching/detaching the MCP, and preventsthe MCP from being damaged and so forth during the operation.

Namely, the present invention relates to an electron multiplier equippedwith a microchannel plate (MCP) having a plurality of channels formultiplying an incident electron and outputting thus multipliedelectron, the MCP being provided with a first electrode on a firstsurface positioned on a side where the electron is incident, and asecond electrode on a second surface opposing the first surface. Theplurality of channels extend along a predetermined reference axis fromthe first surface toward the second surface.

In particular, in order to prevent an electric field from leaking due toa voltage supplied to the MCP acting as an electron multiplying means,the electron multiplier according to the present invention comprises anelectric field shield cap (outer electric field shield member)accommodating the MCP. The electric field shield cap has a body portionsurrounding at least a side face of the MCP while being separatedtherefrom by a predetermined distance. Further, in order for theelectric field to be more effectively restrained from leaking to thefront side of the MCP, the electric field shield cap preferablycomprises a support portion (outer support portion) extending from afirst end part of the body portion toward the reference axis andsupporting the MCP. This support portion has an opening for exposing thefirst surface of the MCP, and this electric field shield member havingthis support portion realizes an MCP support structure and an electricfield shield function at the same time.

Also, the electron multiplier according to the present inventioncomprises a base for mounting the electric field shield capaccommodating the MCP therein, thereby realizing a structure which caneffectively restrain the MCP from being damaged and so forth during anoperation of replacing the MCP and attain a sufficient electric fieldshield effect. Namely, a second end of the electric field shield capopposing a first end thereof (an end part provided with the supportportion) is detachably mounted to the base, thereby realizing astructure in which the base and the electric field shield cap areelectrically connected to each other.

In this configuration, as the electric field shield cap is groundedwhile being electrically insulated from the first electrode formed onthe first surface of the MCP or while being electrically in contact withthe first electrode through a predetermined insulating member, it isshielded from the electric field occurring when a voltage is supplied tothe MCP, whereby the electric field can be effectively restrained fromleaking to the front side of the MCP (the space on the side where theelectron source is installed with respect to the MCP) (i.e., theinfluence of the electric field on the electron beam orbit can beeliminated). When the electric field shield cap and the first surface ofthe MCP (or the first electrode formed on the first surface) areelectrically insulated from each other, the first electrode can be setto a potential other than the ground potential.

The electron multiplier according to the present invention may furthercomprise an intermediate member (inner electric field shield member)accommodated within a space defined by the electric field shield capwhile being electrically insulated from the electric field shield capwhile being electrically in contact with the second surface of the MCP.This member is an electrically conductive member extending along thebody portion of the electric field shield cap and having an opening forexposing the second surface of the MCP. More preferably, theintermediate member comprises a support portion (inner support portion),provided at a first end thereof electrically connected to the secondelectrode on the MCP, for holding the MCP in cooperation with thesupport portion of the electric field shield cap.

Thus, the electric field shield cap electrically connected to the firstelectrode disposed on the first surface of the MCP functions as an outerelectrode of the MCP, whereas the electrically conductive intermediatemember (inner electric field shield member) electrically connected tothe second electrode provided on the second surface of the MCP functionsas an inner electrode of the MCP. Since this structure holds the firstand second surfaces of the MCP by the respective support portions ofthese two electrically conductive members, the attaching/detachingoperation is easy, the MCP is prevented from being damaged during theoperation, and the operation of replacing the MCP can easily be carriedout.

In addition, in accordance with the present invention, since theelectric field shield cap and the base are electrically connected toeach other and grounded, the electric field can be securely preventedfrom leaking out of the electron multiplier, whereby the potentialchange caused by power feed switching would not reach the outside andwould not affect the electron beam orbit. Consequently, when theelectron multiplier is employed in an electron microscope, the deviationof the transmitted electron image and reflected electron image from eachother in the field of view can be eliminated.

As a structure for securing the MCP to the electric field shield cap, asecuring structure by a predetermined adhesive, a mechanical screwingstructure, or the like, for example, is employable in the electronmultiplier according to the present invention.

When securing the MCP to the electric field shield cap with an adhesive,the electron multiplier comprises a first electrically conductiveadhesive, provided between the support portion of the electric fieldshield cap and the first surface of the MCP, for securing the MCP to theelectric field shield cap; a second electrically conductive adhesive,provided between the support portion of the electrically conductiveintermediate member and the second surface of the MCP, for securing theelectrically conductive intermediate member to the MCP; and aninsulating adhesive, provided in a space between a side face of the MCPand the body portion of the electric field shield cap, for securing theelectrically conductive intermediate member to the electric field shieldcap while the intermediate member and the electric field shield cap areelectrically insulated from each other.

As another securing structure, an insulating member may be arrangedwithin a space defined by a side face of the MCP, the support portion ofthe electric field shield cap, and the support portion of theelectrically conductive intermediate member, such that the supportportion of the electric field shield cap, the insulating member, and thesupport portion of the electrically conductive intermediate member aremechanically secured with an insulating screw penetrating through eachthereof. According to this structure, the MCP can be held by therespective support portions of the electric field shield cap andelectrically conductive intermediate member easily and reliably.

The electrically conductive intermediate member accommodated in theelectric field shield cap may be configured so as to be mounted eitherdirectly or indirectly to the base attached to the second end of theelectric field shield cap. Namely, in the structure in which theelectrically conductive intermediate member is mounted onto the mainsurface of a circuit board which has already been attached to the base,mounting the second end of the electric field shield cap to the base caneasily realize the structure for holding the MCP by the support portionof the electric field shield cap and the support portion of theelectrically conductive intermediate member. Also, when replacing theMCP, it is unnecessary to replace the electrically conductiveintermediate member, whereby the cost of the replaceable MCP portion canbe cut down.

In the structure in which the insulating member is provided between thesupport portion of the electric field shield cap and the first surfaceof the MCP in the electron multiplier according to the presentinvention, on the other hand, the first electrode provided on the firstsurface of the MCP and the electric field shield cap are electricallyinsulated from each other, whereby they can be set to their respectivepotentials different from each other. Here, the insulating member isprovided with an opening for exposing the first surface of the MCP.Also, in this configuration, the electrically conductive intermediatemember accommodated within the electric field shield cap can be designedso as to have a ring-like form, so that the distance between the MCP andan electron reflector attached, through an insulating member, to thesecond end of the electrically conductive intermediate member (the endpart of the electrically conductive member opposing the first endelectrically in contact with the second electrode formed on the secondsurface of the MCP) can be shortened, thus enabling efficient electronmultiplication.

In addition, the electron multiplier according to the present inventioncomprises a circuit board disposed between an electron reflectorattached, through the insulating member, to the second end of theelectrically conductive intermediate member and the base, the boardhaving a spring electrode for pressing the electron reflector againstthe second end of the electrically conductive intermediate member, sothat assembling steps such as securing operations and the like can besimplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the overall configuration of anelectron microscope to which an electron multiplier according to thepresent invention is applied;

FIG. 2 is a sectional view showing the schematic configuration(including a voltage supply system) of a first embodiment of theelectron multiplier according to the present invention in the statewhere it is employed in the electron microscope shown in FIG. 1;

FIG. 3 is a plan and assembling view showing a major part of theelectron multiplier shown in FIG. 2, wherein each member is illustratedby a sectional view taken along the line I—I of the plan view;

FIG. 4 is a view, identical to the plan and assembling view shown inFIG. 3, showing the major part of the electron multiplier according tothe present invention, whose lines are partly omitted in order toclarify the cross-sectional form of each member;

FIG. 5 is a sectional view showing the schematic configuration of thefirst embodiment of the electron multiplier according to the presentinvention;

FIG. 6 is a view showing the structure of a circuit board incorporatedin the electron multiplier shown in FIG. 5, whose individual sectionsshow a plan view of the circuit board seen from the microchannel plate(MCP) side, a side view of the circuit board, and a plan view of thecircuit board seen from the base side, respectively;

FIG. 7 is a view showing a state where an electric field shield cap(outer electric field shield member) is mounted to a base in theelectron multiplier according to the present invention;

FIG. 8 is a view for explaining the state of an electric field generatedin the vicinity of an MCP without electric field shielding;

FIG. 9 is a view for explaining an electric shield effect caused by anelectric field shield cap (outer electric field shield member) in theelectron multiplier according to the present invention;

FIG. 10 is a sectional view showing the configuration of a secondembodiment of the electron multiplier according to the presentinvention;

FIG. 11 is a view showing the configuration of the circuit boardemployed in the electron multiplier of FIG. 10, whose individualsections show a plan view of the circuit board seen from the MCP side, aside view of the circuit board, and a plan view of the circuit boardseen from the base side, respectively;

FIG. 12 is a sectional view showing the schematic configuration(including a voltage supply system) of a third embodiment of theelectron multiplier according to the present invention in the statewhere it is employed in the electron microscope shown in FIG. 1;

FIG. 13 is a plan and assembling view showing a major part of theelectron multiplier shown in FIG. 12, wherein each member is illustratedby a sectional view taken along the line II—II of the plan view;

FIG. 14 is a view, identical to the plan and assembling view shown inFIG. 13, showing the major part of the electron multiplier according tothe present invention, whose lines are partly omitted in order toclarify the cross-sectional form of each member;

FIG. 15 is a sectional view showing a structure for holding the MCP inthe third embodiment of the electron multiplier according to the presentinvention;

FIG. 16 is a schematic view showing the state of an electric fieldwithin an electrically conductive intermediate member (inner electricfield shield member) having an outward support portion;

FIG. 17 is a schematic view showing the state of an electric fieldwithin an electrically conductive intermediate member (inner electricfield shield member) having an inward support portion;

FIG. 18 is a sectional view (of a fourth embodiment) showing theelectron multiplier in which an electrically conductive intermediatemember (inner electric field shield member) is disposed on the baseside;

FIG. 19 is a plan and sectional view for showing a structure forsecuring the MCP by screws made of an insulating material, in which thesectional view shows the cross section taken along the line III—III ofthe plan view;

FIG. 20 is a view showing an assembling step of the structure forsecuring the MCP by the insulating screws shown in FIG. 19;

FIG. 21 is a view, identical to the plan and assembling view shown inFIG. 19, whose lines are partly omitted in order to clarify thecross-sectional form of each member;

FIG. 22 is a view, identical to the plan and assembling view shown inFIG. 20, whose lines are partly omitted in order to clarify thecross-sectional form of each member;

FIG. 23 is a plan and sectional view showing the configuration of amajor part of a fifth embodiment of the electron multiplier according tothe present invention, in which the sectional view shows the crosssection taken along the line IV—IV of the plan view;

FIG. 24 is a view showing an assembling step of the major part of thefifth embodiment shown in FIG. 23;

FIG. 25 is a view, identical to the plan and assembling view shown inFIG. 23, whose lines are partly omitted in order to clarify thecross-sectional form of each member; and

FIG. 26 is a view, identical to the plan and assembling view shown inFIG. 24, whose lines are partly omitted in order to clarify thecross-sectional form of each member.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, the electron multiplier according to the presentinvention will be explained with reference to FIGS. 1 to 26. In eachembodiment, a case where the electron multiplier is employed in anelectron microscope will be explained as an example. Among the drawings,the same numerals or letters indicate the same or equivalent parts,without repeating their overlapping explanations.

FIG. 1 is a view showing the overall configuration of an electronmicroscope employing the electron multiplier according to the presentinvention. The electron microscope 10 shown in FIG. 1 is a reflectiontype viewing apparatus with which a reflected electron image of a sample2 can be observed, whose one end is provided with an electron gun 11(electron source) for emitting an electron beam B, whereas the other endis provided with a vacuum chamber 12 adapted to be evacuated as needed.Disposed within the vacuum chamber 12 are a movable table 13 on which anelectron multiplier 1 is installed, and an electron detector 14.

The movable table 13 provided with the electron multiplier 1 holding thesample 2 on its surface facing the electron gun 11 is adapted to movethe electron multiplier 1 arbitrarily along horizontal axes (X and Yaxes) orthogonal to each other and can arbitrarily be tilted about ahorizontal axis (T axis). Also, the movable table 13 is rotatable abouta vertical axis (R axis) passing through the center position.

The electron detector 14 is an apparatus which detects an electron fromthe sample 2 and converts thus detected electron into an electricsignal. For example, it is constituted by a collector 14 a forcollecting the electron to be detected, a scintillator 14 b which emitsfluorescence when the electron is made incident thereon, and aphotomultiplier tube 14 c which generates a photoelectron in response tothe fluorescence and outputs it as a multiplied electric signal.

In such an electron microscope, the electron beam B emitted from theelectron gun 11 is converged by a predetermined electron lens 15disposed between the electron gun 11 and the vacuum chamber 12, and isfurther scanned by a deflection coil 16, so as to irradiate the surfaceof the electron multiplier 1 (on which the sample 2 is installed) set onthe movable table 13. The secondary electron generated by the electronbeam B irradiating the sample 2 is detected by the electron detector 14and is outputted as an electric signal therefrom. The detected electricsignal is further amplified by an amplifier 17 and is fed to a CRT 18 asa video signal. At this time, as the deflection coil of the CRT 18 issynchronized with the deflection coil 16 for the electron beam B by ascanning power supply 19, an enlarged reflected electron image of thesample 2 is displayed on the CRT 18.

A first embodiment of the electron multiplier 1 according to the presentinvention will now be explained with reference to FIGS. 2 to 7. FIG. 2is a sectional view showing the schematic configuration (including avoltage supply system) of the first embodiment of the electronmultiplier according to the present invention in the state where it isemployed in the electron microscope shown in FIG. 1. FIG. 3 is a planand assembling view showing a major part of the electron multiplier 1shown in FIG. 2, wherein each member is illustrated by a sectional viewtaken along the line I—I of the plan view. FIG. 4 is a view, identicalto the plan and assembling view shown in FIG. 3, showing the major partof the electron multiplier according to the present invention, whoselines are partly omitted in order to clarify the cross-sectional form ofeach member. FIG. 5 is a sectional view showing the schematicconfiguration of the first embodiment of the electron multiplieraccording to the present invention. FIG. 6 is a view showing thestructure of a circuit board incorporated in the electron multipliershown in FIG. 5, whose individual sections show a plan view of thecircuit board seen from the microchannel plate (MCP) side, a side viewof the circuit board, and a plan view of the circuit board seen from thebase side, respectively. FIG. 7 is a view showing a state where anelectric field shield cap (outer electric field shield member) ismounted to a base in the electron multiplier according to the presentinvention.

As shown in FIG. 2, the electron multiplier 1 is installed on themovable table 13 within the electron microscope 10. Disposed in theupper part of main body portion of the electron multiplier 1 is amicrochannel plate (MCP) 3 having a plurality of channels 31 forelectron multiplication extending along a predetermined axis AX (axisaligning with the emitting direction of the electron beam B) from afirst surface facing the electron gun 11 toward a second surfaceopposing the first surface. The MCP 3 is mounted at a first end of aring body 6 acting as an electrically conductive intermediate membermade of an electrically conductive member such as copper, whereas anelectron reflector 7 is attached, through a ring-shaped insulatingmember 5 made of glass or the like, to a second end of the ring body 6opposing the first end. Here, the ring body 6 and the insulating member5 are formed with their respective through holes 6 b, 5 a fortransmitting therethrough electrons from the MCP 6 to the electronreflector 7 or vice versa.

A first electrode 30 a is formed on the first surface of the MCP 3facing the electron gun 11, whereas a second electrode 30 b is formed onthe whole second surface opposing the first surface. The first andsecond electrodes 30 a, 30 b are formed by vapor deposition of Inconel,Ni—Cr alloy, or the like on each of the main surfaces (first and secondsurfaces) of the MCP 3.

The MCP 3 not only functions as a mount for installing the sample 2 tobe observed, but also functions to multiply electrons passing througheach channel 31 from the first surface toward the second surface.Namely, the inner wall of each of the numerous channels 31 within theMCP 3 is formed with a film made of a material adapted to releasesecondary electrons. When the MCP 3 is employed in a reflection typeelectron microscope, as a voltage is applied thereto such that thepotential at the electron release surface (the first surface of the MCP3 formed with the first electrode 30 a as the output electrode) becomeshigher than that at the electron entrance surface (the second surface ofthe MCP 3 formed with the second electrode 30 b as the input electrode),electrons enter the channels 31 of the entrance surface and exit fromthe release surface after being multiplied as being reflected by theinner walls of the channels 31.

Also, as shown in FIG. 2, the electron multiplier 1 is provided with theelectron reflector 7, separated from the MCP 3 by a predetermineddistance, on the side opposite the space where the electron gun 11 isinstalled. The surface of the electron reflector 7 facing the MCP 3 isformed with a reflecting surface 71, which reflects the electron beam Bpassed through interstices between pieces of the sample 2 or through thesample 2 and further through the individual channels 31 of the MCP 3.

The electron multiplier 1 further comprises an electric field shield cap20 (outer electric field shield member) which comes into contact withthe peripheral portion of the first surface of the MCP 3 formed with thefirst electrode and surrounds the side face of the MCP 3 while beingseparated therefrom by a predetermined distance. The electric fieldshield cap 20 acts to prevent the electric field from leaking to thefront side of the MCP 3, and is made of a metal, alloy, or the like,such as stainless steel, for example. Here, as shown in FIGS. 3 and 4,the electric field shield cap 20 is constituted by a body portion 20 bextending along the reference axis AX (aligning with the advancingdirection of the electron beam B) so as to surround the reference axisAX, and a support portion 20 a extending from the first end of the bodyportion 20 b toward the reference axis AX and having an opening 20 c forexposing the first surface of the MCP 3.

Further, as can be seen from FIGS. 3 to 5, the electron reflector 7having the reflecting surface 71 is attached, through the insulatingmember 5, to the second end of the ring body 6 opposing the first end.Installed between the electron reflector 7 and a base 4 is a circuitboard 21 for supplying a predetermined voltage to each of theabove-mentioned parts. As shown in FIG. 6, the front face of the circuitboard 21 is provided with leaf springs 40, 41 (spring electrodes).Connected to the rear face of the circuit board 21 is a bleeder resistor42 which subjects the voltage applied between a ring-shaped wire 43formed at the periphery of lower surface of the circuit board 21 and theleaf spring 41 and the voltage applied between the ring-shaped wire 43and a pad portion 41 a to resistance division, so as to set them todifferent potentials. For example, in the case explained later, thering-shaped wire 43 is grounded, whereas the leaf spring 41 and the padportion 41 a are set to −800 V and −700 V, respectively. Here, the leafspring 41 is integrally formed through the upper and lower faces of thecircuit board 21, whereas the pad portion 41 a and the leaf spring 40are conducting to each other through the upper and lower faces of thecircuit board 21. Also, the circuit board 21 is connected to an externalcircuit (see FIG. 2), whereby a predetermined voltage is suppliedthereto. Further, the circuit board 21 is provided with a vent hole 21 apenetrating through the upper and lower face thereof.

The ring-shaped wire 43 is electrically connected to the base 4, and thefirst electrode 30 a formed on the first surface of the MCP 3 isgrounded through the electric field shield cap 20 mounted to the base 4.The leaf spring 41 supplies a voltage (−800 V) to the electron reflector7 from the lower face of the circuit board 21; whereas the leaf spring40 comes into contact with the ring body 6, thereby supplying a voltage(−700 V), through the ring body 6, to the second electrode 30 b formedon the second surface. Therefore, the leaf spring 41 of the circuitboard 21 functions to press the electron reflector 7 against the secondend of the ring body 6 through the insulating member 5 inevitably whenthe electric field shield cap 20 is mounted to the base 4 with thecircuit board 21 being installed at a predetermined position of the base4 (see FIG. 7).

The individual members of the electron multiplier 1 according to thefirst embodiment are assembled as follows. Namely, the electronmultiplier 1 is constituted by three kinds of parts, i.e., the electricfield shield cap 20; an MCP portion composed of the MCP 3 and the ringbody 6; and a sample mount portion composed of the insulating member 5,the electron reflector 7, the circuit board 21 made of a glass epoxyresin or the like, and the base 4. Among them, in the MCP portion, theMCP 3 and the ring body 6 are bonded to each other with an electricallyconductive adhesive, for example, such as silver paste, carbon paste,indium paste, solder, or the like. Also, the sample mount portion isprepared as being fixed with such an electrically conductive adhesiveand an insulating adhesive such as an epoxy resin adhesive, for example.

Here, the MCP portion may be made separately replaceable from theelectron multiplier in order to cut down the cost of the replaceablepart. Also, the MCP portion is not restricted to the one constituted bythe MCP 3 and the ring body 6, but may have a configuration solelyconstituted by the MCP 3, a configuration including the insulatingmember 5 in addition to the MCP 3 and the ring body 6, or aconfiguration including the insulating member 5 and the electronreflector 7 in addition to the MCP 3 and the ring body 6. Various kindsof such combinations are possible depending on the easiness in handlingwhen replacing the MCP portion, the cost balance of the parts to bereplaced, and the like, whereas the MCP portion can be replaced as asingle unit. Without being restricted to solder and adhesives, screwingor the like may similarly be employed for bonding the individualconstituent members as will be explained later.

Further, as indicted by part A in FIG. 7, the electric field shield cap20 can easily be attached to the base 4 by causing a fitting portion 44of the electric field shield cap 20 (disposed at the second end opposingthe first end having the support portion 20 a) to fit a fitting pin 45disposed at the table 4. Here, the electric field shield cap 20 and thering body 6 are provided with their respective vent holes 20 e, 6 a.

As shown in FIG. 2, a voltage supply system 9 is connected to the MCP 3and the electron reflector 7. While FIG. 2 illustrates connection linesonly schematically, voltages are more specifically supplied by thevoltage supply portion formed on the rear face of the circuit board 21as shown in the above-mentioned FIG. 6.

The voltage supply system 9 comprises a variable power source 91 forapplying a predetermined voltage between the first electrode 30 a andsecond electrode 30 b of the MCP 3, and a variable power source 92 forapplying a predetermined voltage between the second electrode 30 b ofthe MCP 3 and the electron reflector 7. Also, as switches 93, 94 areselectively operated, viewing modes of the electron microscope 10employing the electron multiplier 1 can be switched. Namely, atransmitted electron image of the sample 2 is obtained when a voltage issupplied to the electron reflector 7 and the MCP 3, whereas a reflectedelectron image of the sample 2 is obtained when the electron reflector 7and the MCP 3 are placed in a grounded state with no voltage beingsupplied thereto.

Operations of the electron microscope 10 equipped with the electronmultiplier 1 according to the present invention will now be explained.

First, as shown in FIG. 2, the sample 2 to be observed is mounted on thefirst surface of the MCP 3 where the first electrode 30 a is formed. Themethod of holding the sample 2 at the MCP 3 includes upper facefixation, interior embedding, intimate lower face contacting, and thelike, which may be selectively employed depending on the material of thesample 2 and the viewing mode.

While carrying the sample 2, the MCP 3 is mounted on the ring body 6 ofthe electron multiplier 1. Subsequently, the electric field shield cap20 is mounted to the base 4 so as to cover the MCP 3. Namely, in theelectric field shield cap 20, the support portion 20 a extending fromthe first end toward the reference axis AX is electrically connected tothe first electrode formed on the first surface of the MCP 3 and coversthe side face of the MCP 3. On the other hand, the second end of theelectric field shield cap 20 is attached to the base 4, whereas both ofthe electric field shield cap 20 and the base 4 are grounded while beingelectrically connected to each other.

Thus configured electron multiplier 1 is set within the reflection typeelectron microscope 10. Namely, the main body portion (the portionexcluding the voltage supply system 9) of the electron multiplier 1 isplaced on the movable table 13 disposed within the vacuum chamber 12 inthe electron microscope 10. Then, air is evacuated from the vacuumchamber 12, so that the latter attains a vacuum state therein.

Referring to a case where a transmitted electron image of the sample 2is observed, the switches 93, 94 are selectively operated so as tosupply voltages from the voltage supply system 9 such that the firstelectrode 30 a disposed on the first surface (the surface to beirradiated with the electron beam B) of the MCP 3 is grounded, while avoltage of about −700 V is applied to the second electrode 30 b disposedon the second surface of the MCP 3. Also, a voltage of about −800 V isapplied to the electron reflector 7 such that its potential becomeslower than that of the second surface 30 b of the MCP 3 by 100 V. Alongtherewith, predetermined voltages are applied to the collector 14 a ofthe electron detector 14 located on the electron gun 11 side and itsscintillator 14 b such that they have potentials of +100 V and +10 kV,respectively, and the electron beam B is emitted toward the sample 2 onthe MCP 3 as shown in FIGS. 1 and 2.

Of the electron beam B, a part irradiating locations other than thesample 2 or irradiating thinner portions of the sample 2 passes throughinterstices between pieces of the samples 2, i.e., through portions freeof the sample 2, or through the sample 2, and then through theindividual channels 31 within the MCP 3, thereby irradiating thereflecting surface 71 of the electron reflector 7. A part of theelectron beam B irradiating the reflecting surface 71 is furtherreflected by the reflecting surface 71 as a reflected electron andgenerates a secondary electron.

Here, if the reflecting surface 71 has a protrusion or depression formedthereon, then the reflected and secondary electrons are released fromthe reflecting surface 71 in a diffused state. The reflected andsecondary electrons from the reflecting surface 71 are reliably guidedtoward the MCP 3 whose potential is higher by 100 V.

Subsequently, the reflected and secondary electrons enter the individualchannels 31 of the MCP 3 again, are multiplied every time they arereflected by the inner wall of the channel 31, and then are releasedfrom the first surface of the MCP 3 through the first electrode 30 a.Here, since the sample 2 on the MCP 3 is irradiated with the reflectedand secondary electrons from the electron reflector 7, the sample 2positively charged upon irradiation of the electron beam B isneutralized, so as to keep its charge from increasing. The reflected andsecondary electrons released from the first surface of the MCP 3 areguided to the collector 14 a of the electron detector 14 set at apotential higher than that of the first electrode 30 a, and are madeincident on the scintillator 14 b. The fluorescence from thescintillator 14 b generated by the electrons made incident thereon isdetected by the photomultiplier 14 c as an electronic signal.

Though the electron beam B irradiating the sample 2 is reflected by thesurface thereof and generates secondary electrons, they would not bemultiplied by the MCP 3, and the amount thereof is much smaller thanthat of the multiplied reflected and secondary electrons from theabove-mentioned reflecting surface 71 (on the order of {fraction(1/1000)} to {fraction (1/10000)}), whereby they are only negligiblydetectable by the electron detector 14.

The electric signal thus obtained by the electron detector 14 isamplified by the amplifier 17 and is fed as a video signal into the CRT18 scanned in synchronization with the electron beam B, thereby beingdisplayed on the CRT 18 as a transmitted electron image of the sample 2.Thus obtained transmitted electron image is displayed such that the partwhere the sample 2 is so thick that electrons cannot pass therethroughis dark, the part free of the sample 2 is bright, the part where sample2 is so thin that the electrons partly pass therethrough has anintermediate brightness. As mentioned above, since the part where thesample 2 exists has a signal strength on the order of {fraction(1/1000)} to {fraction (1/10000)} that in the other parts, an electronimage having a very clear contrast is obtained. Also, since thetransmitted electron image is clear even when the amount of irradiationof the electron beam B is reduced, the amount of irradiation of theelectron beam can be lowered so as to narrow the electron beam, therebyyielding a transmitted electron image with a high resolution.

Here, a case where the above-mentioned transmitted electron image isobserved by means of an electron multiplier not equipped with theelectric field shield cap 20 covering at least the side face of the MCP3 will be explained. FIG. 8 is a schematic view showing the state ofelectric field in the electron multiplier not provided with the electricfield shield cap 20, whereas FIG. 9 is a schematic view showing thestate of electric field in the electron multiplier 1 provided with theelectric field shield cap 20.

As can be seen from FIG. 8, in the case where the electric field shieldcap 20 is not provided, when a predetermined voltage is applied to theMCP 3 and the electron reflector 7, the MCP 3 leaks out of the MCP 3 asindicated by equipotential lines in FIG. 8. Consequently, thus leakedelectric field changes the orbit of the electron beam incident on theelectron microscope, thereby yielding a deviation in field of viewbetween the transmitted electron image and the reflected electron image.

As shown in FIG. 9, on the other hand, in the electron multiplier inwhich the electric field shield cap 20 is mounted such that its supportportion 20 a is in contact with the first electrode disposed on thefirst surface of the MCP 3 and covers the side face of the MCP 3, whileit also comes into contact with the base 4, the electric field can beprevented from leaking, as indicated by equipotential lines therein.Namely, in the MCP 3 and the electron reflector 7, the potential changecaused by power feed switching would not reach the outside of theelectron multiplier, and would not affect the electron beam orbit,whereby the deviation in field of view between the transmitted electronimage and the reflected electron image can effectively be reduced.Consequently, there would be no fear of losing sight of the object to beobserved, or it becomes unnecessary to correct the deviation in field ofview. In the case where the transmitted electron image and the reflectedelectron image are displayed as being superposed on each other, theeffects mentioned above are large in particular.

Referring to a case where a reflected electron image of the sample 2 isobserved, on the other hand, it can easily be observed when the switches93, 94 of the voltage supply system 9 shown in FIG. 9 are selectivelyoperated. Namely, the switches 93, 94 are operated such that the MCP 3and the electron reflector 7 are grounded (voltage application isstopped). As a consequence, the multiplying function of the MCP 3 andthe reflecting function of the electron reflector 7 are ceased. When theelectron beam B irradiates the sample 2 on the MCP 3 in this state, apart of the electron beam B which does not irradiate the sample 2 is notdetected by the electron detector 14, and only the secondary electronsgenerated at the surface of the sample 2 are detected by the electrondetector 14. Therefore, when the gain of the amplifier 17 isappropriately adjusted, it is possible to observe a reflected electronimage which can be obtained with a conventional reflection type electronmicroscope.

FIG. 10 is a sectional view showing the configuration of a secondembodiment of the electron multiplier according to the presentinvention. FIG. 11 is a view showing the configuration of the circuitboard employed in the electron multiplier of FIG. 10, whose individualsections show a plan view of the circuit board seen from the MCP side, aside view of the circuit board, and a plan view of the circuit boardseen from the base side, respectively.

While the first electrode 30 a provided on the first surface of the MCP3 and the electric field shield cap 20 are electrically connected toeach other and are individually grounded in the electron multiplier ofthe first embodiment shown in FIG. 5; the first electrode 30 a disposedon the first surface of the MCP 3 and the electric field shield cap 20are electrically separated from each other by a spacer 50 made of aninsulating material in the electron multiplier of the second embodiment,so that they can be set to their respective potentials different fromeach other. Except for this feature, the second embodiment is basicallysimilar to the first embodiment shown in FIG. 5.

Namely, as shown in FIG. 10, a glass epoxy substrate 50, which is aninsulating member, is mounted at the peripheral portion of the firstsurface of the MCP 3, whereas the support portion 20 a of the electricfield shield cap 20 is installed on the glass epoxy substrate 50.Connected to the rear face of the circuit board 21 is a bleeder resistor42, by which potentials of −800 V, −30 V, and −700 V are set between apad portion 54 and the leaf spring 41, and between the pad portion 54and pins 52, 53 formed on the surface of the circuit board 21,respectively.

Here, the part of the glass epoxy substrate 50 coming into contact withthe peripheral portion of the MCP 3 is covered with an electricallyconductive coating 50 a. Byway of the electrically conductive coating 50a, a voltage is supplied from the circuit board 21 via the pin 52,whereby a potential of −30 V is set at the first surface (firstelectrode) of the MCP 3. Here, a voltage on the order of −100 V to +100V can be set at the first surface of the MCP 3, and thus set voltage canappropriately be changed as the bleeder resistance ratio is adjusted,for example.

The electric field shield cap 20 is conducting and grounded, through thebase 4, from the pad portion 54 formed on the rear face of the circuitboard 21. Further, the peripheral portion of the second surface of theMCP 3 is provided with an electrically conductive ring-shaped member 51,which is electrically connected to the second electrode disposed on thesecond surface of the MCP 3. The voltage supplied from the circuit board21 is fed, through the pin 53 and ring-shaped member 51, to the secondelectrode disposed on the second surface of the MCP 3, whereby thesecond surface is set to −700 V. A voltage of −800 V is supplied to theelectron reflector 7 through the leaf spring 41.

When the set voltage of the first surface of the MCP 3 is adjusted to avalue different from the ground voltage, then the reflected andmultiplied electrons released from the MCP 3 can be collected by theelectron detector 14 more efficiently.

Also, in the second embodiment, as the ring-shaped member 51 is used inplace of the ring body 6 extending along the reference axis AX so as tosurround the latter, the distance between the MCP 3 and the electronreflector 7 can be shortened, thus enabling more efficient electronmultiplication.

The configuration of a third embodiment of the electron multiplieraccording to the present invention will now be explained with referenceto FIGS. 12 to 15. FIG. 12 is a sectional view showing the schematicconfiguration (including a voltage supply system) of the thirdembodiment of the electron multiplier according to the present inventionin the state where it is employed in the electron microscope shown inFIG. 1. FIG. 13 is a plan and assembling view showing a major part ofthe electron multiplier shown in FIG. 12, wherein each member isillustrated by a sectional view taken along the line II—II of the planview. FIG. 14 is a view, identical to the plan and assembling view shownin FIG. 13, showing the major part of the electron multiplier accordingto the present invention, whose lines are partly omitted in order toclarify the cross-sectional form of each member. FIG. 15 is a sectionalview showing an example of structure for holding the MCP in the thirdembodiment of the electron multiplier according to the presentinvention.

The electron multiplier 100 according to the third embodiment isinstalled on the movable table 13 within the vacuum chamber 12 of theelectron microscope 10 as shown in FIG. 1. In the upper part of the mainbody portion of the electron multiplier 100, an MCP 3 is installed. Theinner face of an outer support portion 200 a extending from the firstend of an outer electric field shield member 200 made of an electricallyconductive material such as a metal toward the reference axis AX (axisaligning with the advancing direction of the electron beam B) is bondedto the peripheral part of the first surface of the MCP 3 on the sideirradiated with the electron beam B, whereas the outer face of aring-shaped support portion 210 a (inner support portion) extending fromthe first end of an inner electric field shield member 210 made of anelectrically conductive material such as a metal toward the outerelectric field shield member 200 is bonded to the peripheral part of thesecond surface of the MCP 3, these support portions being integrallyconstructed as shown in FIG. 15.

Also, in the third embodiment, a first electrode 30 a is formed on thefirst surface of the MCP 3, whereas a second electrode 30 b is formed onthe whole second surface opposing the first surface. The first andsecond electrodes 30 a, 30 b are formed by vapor deposition of Inconel,Ni—Cr alloy, or the like on the first and second surfaces of the MCP 3.

Here, as shown in FIG. 14, the outer electric field shield member 200accommodating the MCP 3 is constituted by a body portion 200 dsurrounding at least the side face of the MCP 3 and the outer supportportion 200 a (having an opening 200 c for exposing the first surface ofthe MCP 3) extending from the first end of the body portion 200 d towardthe reference axis AX. Installed in the space within the outer electricfield shield member 200 is the inner electric field shield member 210comprising a body portion 210 b extending along the reference axis AX soas to surround the reference axis AX and the inner support portion 210 a(having an opening 210 c for exposing the second surface of the MCP 3)extending from the first end of the body portion 210 b and holding theMCP 3 in cooperation with the support portion 200 a of the outerelectric field shield member 200.

As the structure for holding the MCP 3, electrically conductiveadhesives 201, 211, for example, such as silver paste, carbon paste,indium paste, solder, and the like, bond the outer electric field shieldmember 200 and the first electrode 30 a of the MCP 3 to each other, andthe inner electric field shield member 210 and the second electrode 30 bof the MCP 3 to each other in order to establish reliable electriccontact. Also, for reinforcement, the outer support portion 200 a andthe inner support portion 210 a are secured to each other through theside face of the MCP 3 with an insulating adhesive 212 such as an epoxyresin type adhesive or the like (see FIG. 15).

As with the above-mentioned first and second embodiments, the MCP 3 notonly functions as a mount for installing the sample 2 to be observed,but also functions to multiply electrons passing through each channel31. Namely, the MCP 3 is provided with numerous channels 31 directedfrom the first surface toward the second surface, whereas the inner wallof each channel 31 is formed with a film made of a material adapted torelease secondary electrons. When the MCP 3 is employed in a reflectiontype electron microscope, as a voltage is applied between the first andsecond electrodes 30 a, 30 b such that the potential at the electronrelease surface (the second surface of the MCP 3 formed with the secondelectrode 30 b) becomes higher than that at the electron entrancesurface (the first surface of the MCP 3 formed with the first electrode30 a), electrons enter the channels 31 from the entrance surface andexit from the release surface after being multiplied as being reflectedby the inner walls of the channels 31.

For protecting the MCP 3 and facilitating attachment to the base 4, theouter electric field shield member 200 has a cylindrical form. Here, theouter support portion 200 a of the outer electric field shield member200 is provided with a stepped portion 200 b for facilitating thepositioning for bonding the MCP 3 thereto. Similarly, the inner electricfield shield member 210 has a cylindrical form with the inner supportportion 210 a so as to make it easier to establish electric contact witha leaf spring 290 a of a circuit board 220 set at the base 4 andfacilitate the bonding of the MCP 3.

Also, as shown in FIG. 12, an electron reflector 700 is disposed betweenthe MCP 3 and the base 4 so as to be separated from the MCP 3 by apredetermined distance. The surface of the electron reflector 700 facingthe MCP 3 is formed with a reflecting surface 710, which reflects theelectron beam B passed through interstices between pieces of the sample2 or through the sample 2 and further through the individual channels 31of the MCP 3.

As with the above-mentioned first and second embodiments, a voltagesupply system power feed means 9 is connected to the MCP 3 and theelectron reflector 700. The voltage supply system 9 comprises a variablepower source 91 for applying a predetermined voltage between the firstand second electrodes 30 a, 30 b of the MCP 3, and a variable powersource 92 for applying a predetermined voltage between the secondelectrode 30 b of the MCP 3 and the electron reflector 700. Also, asswitches 93, 94 in the voltage supply system 9 are selectively operated,viewing modes of the electron microscope 10 can be switched. Namely, atransmitted electron image of the sample 2 is obtained when apredetermined voltage is supplied to the electron reflector 700 and theMCP 3, whereas a reflected electron image of the sample 2 is obtainedwhen the electron reflector 700 and the MCP 3 are grounded.

Operations of the electron microscope equipped with the electronmultiplier 100 according to the third embodiment, i.e., those in any ofthe cases where the transmitted electron image and reflected electronimage of the sample 2 are observed, are similar to those in the electronmicroscope equipped with the electron multiplier of the first or secondembodiment.

In the above-mentioned third embodiment, the first end of the innerelectric field shield member 210 secured to the MCP 3 is provided withthe outward inner support portion 210 a. In this case, as indicated byequipotential lines in FIG. 16, the electric field is disturbed in thevicinity of the inner wall of the inner electric field shield member210, thus leaving a relatively narrow region 230 where no electric fielddisturbance occurs. Therefore, as shown in FIG. 17, when the first endof the inner electric field shield 210 is provided with an inwardsupport portion 210 c, then the substantial outside diameter of theinner electric field shield member 210 can be enhanced, whereby theregion 230 free of the electric field disturbance can be secured morewidely as indicated by equipotential lines therein.

Also, as shown in FIG. 18 (a fourth embodiment of the electronmultiplier according to the present invention), the inner electric fieldshield member 210 can be secured beforehand, by means of a screw or thelike, to a circuit board 220 disposed on a base 400. In this case, theMCP 3 is integrated with the outer shield member 200 alone, whereby itis unnecessary to replace the inner electric field shield member 210when replacing the MCP 3. As a consequence, the cost of replaceableparts of the MCP can be cut down.

Further, FIGS. 19 and 20 show structures for holding the MCP 3employable in place of adhesives. Namely, FIG. 19 is a plan andsectional view for showing a structure for securing the MCP by screwsmade of an insulating material, in which the sectional view shows thecross section taken along the line III—III in the plan view. FIG. 20 isa view showing an assembling step of the structure for securing the MCPby the insulating screws shown in FIG. 19. FIG. 21 is a view, identicalto the plan and assembling view shown in FIG. 19, whose lines are partlyomitted in order to clarify the cross-sectional form of each member.Similarly, FIG. 22 is a view, identical to the plan and assembling viewshown in FIG. 20, whose lines are partly omitted in order to clarify thecross-sectional form of each member.

As shown in FIGS. 19 and 20, when securing the MCP 3 by the outerelectric field shield member 200 and the inner electric field shieldmember 210, an insulating screw such as a ceramic screw 240, forexample, may be used. In this case, in order to assure electricinsulation, washers 250, 270, 280 and a nut 290 are also made ofceramics, and aceramic plate 260 (having an opening 260 a for fittingthe MCP 3) is interposed between the above-mentioned members. As aconsequence, the MCP 3 can be secured simply and reliably. Also, sincethis holding structure does not use any adhesive, no gas occurs thereineven when the electron multiplier 100 is used in vacuum. Further, thisstructure can suitably be employed when the electron multiplier 100 isused in a vacuum container having a small volume.

FIG. 23 is a plan and sectional view showing the configuration of amajor part of a fifth embodiment of the electron multiplier according tothe present invention. Here, the sectional view shows the cross sectiontaken along the line IV—IV of the plan view. FIG. 24 is a view showingan assembling step of the major part of the fifth embodiment shown inFIG. 23. FIG. 25 is a view, identical to the plan and assembling viewshown in FIG. 23, whose lines are partly omitted in order to clarify thecross-sectional form of each member. Also, FIG. 26 is a view, identicalto the plan and assembling view shown in FIG. 24, whose lines are partlyomitted in order to clarify the cross-sectional form of each member.

In these drawings, a glass epoxy substrate 451, which is an insulatingmember, is mounted at the peripheral part of the first surface (providedwith the first electrode) of the MCP 3, whereas an electricallyconductive outer electric field shield member 450 is disposed so as toaccommodate the glass epoxy substrate 451. Also, in place of the innerelectric field shield member, an electrically conductive ring-shapedmember 460 which is to be electrically connected to the second electrodeis disposed at the peripheral part of the second surface (provided withthe second electrode) of the MCP 3.

Here, as with each of the above-mentioned embodiments, the outerelectric field shield member 450 is provided with an outer supportportion, which is formed with an opening 450 a for exposing the firstsurface of the MCP 3, whereas the ring-shaped member 460 is formed withan opening 460 a for exposing the second surface of the MCP 3. Also, inFIGS. 23 to 26 showing the fifth embodiment, the configuration on thebase side (electron reflector, circuit board, and the like) is omitted.

Here, the part of the glass epoxy substrate 451 coming into contact withthe peripheral part of the first surface of the MCP 3 is covered with anelectrically conductive coating 451 a, whereby the first surface of theMCP 3 and the outer electric field shield member 450 are electricallyinsulated from each other. As a consequence of this configuration,through the electrically conductive coating 451 a, the first surface ofthe MCP 3 can beset to a potential, other than the ground potential,different from the potential of the outer electric field shield member450. For example, a voltage on the order of −100 V to +100 V can be setat the first surface of the MCP 3, and thus set voltage canappropriately be changed as the bleeder resistance ratio is adjusted,for instance.

When the set voltage of the first surface of the MCP 3 is thus adjusted,the reflected and multiplied electrons released from the MCP 3 can becollected by the electron detector 14 (see FIG. 1) more efficiently.

Also, as the ring-shaped member 460 is used in place of the innerelectric field shield member, the distance between the MCP 3 and the notillustrated electron reflector can be shortened, thus enabling moreefficient electron multiplication.

Though each of the above-mentioned embodiments relates to the case wherethe electron multiplier according to the present invention is employedin the reflection type or transmission type electron microscope, withoutbeing restricted thereto, the present invention is similarly applicableto various kinds of instruments such as energy analyzer and angleanalyzer, while yielding effects similar to those mentioned above.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, since the present invention is mountedwith an electric field shield cap covering at least a side face of anMCP, the potential change caused upon switching the power feed to theMCP would not reach the outside and would not affect the orbit of theelectron beam directed toward the sample installed on the MCP, wherebythe deviation in field of view between the transmitted electron imageand the reflected electron image can be eliminated. As a consequence, itis effective in that there is no fear of losing sight of the object tobe observed, and it is unnecessary to correct the deviation in field ofview. Also, when the electron entrance surface of the MCP facing theelectron source and the electric field shield cap are set to theirrespective potentials different from each other, it is effective incollecting the reflected and multiplied electrons more efficiently.

Further, in accordance with the present invention, since the MCP issandwiched between two electrically conductive members, itsattaching/detaching operation can be carried out quite easily in areliable manner. Also, since the MCP is held by the outer electric fieldshield member (electric field shield cap) and the inner electric fieldshield member accommodated within the outer electric field shieldmember, each positioned outside thereof, the MCP can be prevented frombeing damaged during its replacing operation, whereby the replacingoperation can be carried out easily by any person.

What is claimed is:
 1. An electron multiplier, comprising: amicrochannel plate having a plurality of channels for electronmultiplication, said microchannel plate extending along a predeterminedreference axis from a first surface toward a second surface opposingsaid first surface, said first surface being located on a side on whichan electron is made incident; and an outer electric field shield memberwhich is an electrically conductive member defining a space foraccommodating said microchannel plate, said outer electric field shieldmember having a body portion surrounding at least a side face of saidmicrochannel plate while being separated therefrom by a predetermineddistance.
 2. An electron multiplier according to claim 1, furthercomprising an inner electric field shield member which is anelectrically conductive member electrically connected to the secondsurface of said microchannel plate and is accommodated within the spacedefined by said outer electric field shield member while beingelectrically insulated from said outer electric field shield member,said inner electric field shield member having an opening for exposingthe second surface of said microchannel plate.
 3. An electron multiplieraccording to claim 2, wherein said outer electric field shield membercomprises an outer support portion, extending from a first end of saidbody portion toward said reference axis, for supporting saidmicrochannel plate, said outer support portion having an opening forexposing the first surface of said microchannel plate.
 4. An electronmultiplier according to claim 3, wherein said inner electric fieldshield member comprises an inner support portion for holding saidmicrochannel plate in cooperation with the outer support portion of saidouter electric field shield member.
 5. An electron multiplier accordingto claim 1, further comprising a base mounting said outer electric fieldshield member accommodating said microchannel plate therein, said basebeing electrically connected to said outer electric field shield member.6. An electron multiplier according to claim 2, further comprising anelectron reflector accommodated within the space defined by said outerelectric field shield member, said electron reflector being attached toa second end of said inner electric field shield member opposing a firstend thereof while being electrically insulated from said inner electricfield shield member.
 7. An electron multiplier according to claim 6,further comprising a circuit board accommodated within the space definedby said outer electric field shield member, said board being providedbetween said electron reflector and said base, said board having aspring electrode for pressing said electron reflector against the secondend of said inner electric field shield member.
 8. An electronmultiplier according to claim 4, further comprising: a firstelectrically conductive adhesive, provided between the outer supportportion of said outer electric field shield member and the first surfaceof said microchannel plate, for securing said microchannel plate to saidouter electric field shield member; a second electrically conductiveadhesive, provided between the inner support portion of said innerelectric field shield member and the second surface of said microchannelplate, for securing said inner electric field shield member to saidmicrochannel plate; and an insulating adhesive, provided in a spacebetween the side face of said microchannel plate and the body portion ofsaid outer electric field shield member, for securing said innerelectric field shield member to said outer electric field shield memberwhile said inner and outer electric field shield members areelectrically insulated from each other.
 9. An electron multiplieraccording to claim 4, further comprising: an insulating member providedwithin a space defined by the side face of said microchannel plate, theouter support portion of said outer electric field shield member, andthe inner support portion of said inner electric field shield member;and an insulating screw for securing, at least, said outer supportportion, said insulating member, and said inner support portion whilepenetrating through each thereof.
 10. An electron multiplier accordingto claim 2, further comprising a base mounting said outer electric fieldshield member accommodating said microchannel plate therein, said basebeing electrically connected to said outer electric field shield member,one end of said inner electric field shield member being directly orindirectly secured to said base.
 11. An electron multiplier according toclaim 3, further comprising a spacer, provided between the outer supportportion of said outer electric field shield member and the first surfaceof said microchannel plate, for electrically insulating said outerelectric field shield member and said microchannel plate from eachother, said spacer having an opening for exposing the first surface ofsaid microchannel plate.
 12. An electron multiplier, comprising: amicrochannel plate having a plurality of channels for electronmultiplication, said microchannel plate extending along a predeterminedreference axis from a first surface toward a second surface opposingsaid first surface, said first surface being located on a side on whichan electron is made incident; and an electric field cap as an outerelectric field shield member having a body portion surrounding at leasta side face of said microchannel plate while being separated therefromby a predetermined distance, and a support portion supporting saidmicrochannel plate while being electrically in contact with a firstelectrode provided on the first surface of said microchannel plate, saidbody portion being an electrically conductive member defining a spacefor accommodating said microchannel plate, said support portion being anelectrically conductive member extending from a first end of said bodyportion toward said reference axis and having an opening for exposingthe first surface of said microchannel plate.
 13. An electron multiplieraccording to claim 12, further comprising an intermediate member whichis an electrically conductive member accommodated within the spacedefined by said electric field shield cap while being electricallyinsulated from said electric field shield cap, said intermediate memberholding said microchannel plate in cooperation with the support portionof said electric field shield cap while a first end of said intermediatemember is electrically in contact with a second electrode provided onthe second surface of said microchannel plate, said intermediate memberhaving an opening for exposing the second surface of said microchannelplate.
 14. An electron multiplier according to claim 13, furthercomprising an electron reflector accommodated within the space definedby said electric field shield cap, said electron reflector beingattached to a second end of said intermediate member opposing the firstend thereof while being electrically insulated from said intermediatemember.
 15. An electron multiplier according to claim 14, furthercomprising a base mounting said electric field shield cap accommodatingsaid microchannel plate therein, said base being electrically connectedto said electric field shield cap.
 16. An electron multiplier accordingto claim 15, further comprising a circuit board accommodated within thespace defined by said electric field shield cap, said board beingprovided between said electron reflector and said base, said boardhaving a spring electrode for pressing said electron reflector againstthe second end of said intermediate member.
 17. An electron multiplier,comprising: a microchannel plate having a plurality of channels forelectron multiplication, said microchannel plate extending along apredetermined reference axis from a first surface toward a secondsurface opposing said first surface, said first surface being located ona side on which an electron is made incident; an outer electric fieldshield member which is an electrically conductive member defining aspace for accommodating said microchannel plate, said outer electricfield shield member having a body portion surrounding at least a sideface of said microchannel plate while being separated therefrom by apredetermined distance; and an inner electric field shield member whichis an electrically conductive member electrically connected to thesecond surface of said microchannel plate and is accommodated within thespace defined by said outer electric field shield member while in astate electrically insulated from said outer electric field shieldmember, said inner electric field shield member having an opening forexposing the second surface of said microchannel plate.
 18. An electronmultiplier according to claim 17, wherein said outer electric fieldshield member comprises an outer support portion, extending from a firstend of said body portion toward said reference axis, for supporting saidmicrochannel plate, said outer support portion having an opening forexposing the first surface of said microchannel plate.
 19. An electronmultiplier according to claim 18, wherein said inner electric fieldshield member comprises an inner support portion for holding saidmicrochannel plate in cooperation with the outer support portion of saidouter electric field shield member.
 20. An electron multiplier accordingto claim 19, further comprising: a first electrically conductiveadhesive, provided between the outer support portion of said outerelectric field shield member and the first surface of said microchannelplate, for securing said microchannel plate to said outer electric fieldshield member; a second electrically conductive adhesive, providedbetween the inner support portion of said inner electric field shieldmember and the second surface of said microchannel plate, for securingsaid inner electric field shield member to said microchannel plate; andan insulating adhesive, provided in a space between the side face ofsaid microchannel plate and the body portion of said outer electricfield shield member, for securing said inner electric field shieldmember to said outer electric field shield member while said inner andouter electric field shield members are electrically insulated from eachother.
 21. An electron multiplier according to claim 19, furthercomprising: an insulating member provided within a space defined by theside face of said microchannel plate, the outer support portion of saidouter electric field shield member, and the inner support portion ofsaid inner electric field shield member; and an insulating screw forsecuring, at least, said outer support portion, said insulating member,and said inner support portion while penetrating through each thereof.22. An electron multiplier according to claim 17, further comprising abase mounting said outer electric field shield member accommodating saidmicrochannel plate therein, said base being electrically connected tosaid outer electric field shield member, one end of said inner electricfield shield member being directly or indirectly secured to said base.23. An electron multiplier according to claim 18, further comprising aspacer, provided between the outer support portion of said outerelectric field shield member and the first surface of said microchannelplate, for electrically insulating said outer electric field shieldmember and said microchannel plate from each other, said spacer havingan opening for exposing the first surface of said microchannel plate.